Factory Wheel Specifications - 1973-1987 GM Square Body Trucks
Overview
Understanding factory wheel specifications is crucial for GM Square Body truck owners whether you're restoring to original condition, upgrading for performance, or simply replacing worn wheels. This comprehensive guide covers all factory wheel specifications for C10, C20, K10, K20, K5 Blazer, and Suburban models from 1973-1987.
Factory wheel specifications determine not only the aesthetic appearance of your Square Body but also affect handling, load capacity, brake clearance, and overall vehicle safety. Incorrect wheel specifications can lead to interference issues, poor handling characteristics, and potential safety hazards.
Key areas covered:
- Bolt patterns and hub configurations
- Wheel dimensions and offset specifications
- Load ratings and capacity requirements
- Factory finish options and identification
- Compatibility across model years and variants
Common Issues
Bolt Pattern Confusion
The most common issue owners face is bolt pattern confusion between 5-lug and 6-lug configurations. Many aftermarket wheels are incorrectly advertised or owners assume compatibility without verification.
Offset Problems
Incorrect wheel offset can cause:
- Tire rubbing on suspension components
- Altered scrub radius affecting handling
- Premature wheel bearing wear
- Brake caliper clearance issues (especially with aftermarket brakes)
Load Rating Mismatches
Using wheels with insufficient load ratings on 3/4-ton (C20/K20) applications can result in wheel failure under heavy loads or towing conditions.
Center Bore Issues
Hub-centric vs. lug-centric mounting confusion leads to vibration issues and premature wheel stud wear.
Parts & Materials Needed
Identification Tools
- Ruler or caliper - For measuring wheel dimensions
- Bolt pattern gauge - For accurate bolt circle measurement
- Flashlight - For reading casting marks and part numbers
Reference Materials
GM Part Number Cross-Reference:
- 1973-1980: Various RPO codes (see Year/Model Differences)
- 1981-1987: Updated part numbering system
Replacement Sources
- LMC Truck - Reproduction rally wheels and trim rings
- Classic Industries - OEM-style steel wheels
- Year One - Restored original wheels
- Local tire shops - Often stock basic steel wheels in correct specifications
Step-by-Step Guide
Identifying Your Current Wheels
Step 1: Determine Bolt Pattern
- Count the bolt holes - Should be 5 or 6 for Square Body trucks
- Measure bolt circle diameter:
- 5-lug pattern: Measure center-to-center of opposite studs
- 6-lug pattern: Measure center-to-center of opposite studs
Step 2: Measure Wheel Dimensions
- Diameter: Measure across the wheel from bead seat to bead seat
- Width: Measure between inner bead seats
- Offset: Measure from wheel centerline to mounting surface
- Positive offset: Mounting surface toward front of wheel
- Negative offset: Mounting surface toward back of wheel
Step 3: Locate Identification Markings
Look for stamped numbers on:
- Back of wheel spokes
- Inner rim area
- Center cap area
Common GM wheel codes:
14 x 6JJ - 14" diameter, 6" width, JJ bead profile
15 x 7K - 15" diameter, 7" width, K bead profile
15 x 8K - 15" diameter, 8" width, K bead profile
Determining Correct Specifications
Step 4: Cross-Reference with Vehicle
- Locate RPO code on glove box sticker or build sheet
- Check door jamb sticker for tire size information
- Verify GVW rating to determine load requirements
Year/Model Differences
1973-1980 Early Square Body
Light Duty (C10/K10)
Standard Steel Wheel:
- Size: 15 x 6" or 15 x 7"
- Bolt Pattern: 5 x 5" (5 x 127mm)
- Offset: +3.75" typical
- Center Bore: 3.25"
- Load Rating: 2,000 lbs per wheel
Rally Wheel Option (RPO N67):
- Size: 15 x 7"
- Bolt Pattern: 5 x 5"
- Offset: +4.25"
- Finish: Argent silver with trim ring
- Part Numbers: Various, typically 14010xxx series
Heavy Duty (C20/K20)
Standard Steel Wheel:
- Size: 16 x 6.5"
- Bolt Pattern: 6 x 5.5" (6 x 139.7mm)
- Offset: +3.5"
- Center Bore: 4.25"
- Load Rating: 3,000 lbs per wheel
1981-1987 Late Square Body
Significant Changes for 1981
- Updated wheel designs with revised offset specifications
- New rally wheel style (RPO N67) with different center cap design
- Metric tire sizing adoption affecting wheel specifications
Light Duty (C10/K10)
Standard Steel Wheel:
- Size: 15 x 7"
- Bolt Pattern: 5 x 5"
- Offset: +4.25" (increased from early years)
- Finish: Black or argent depending on trim level
Rally Wheel (RPO N67):
- Size: 15 x 7"
- Distinctive features: Revised spoke design, different center cap
- Part Numbers: 14104xxx series
Aluminum Wheel Option (RPO N90):
- Available: 1982-1987
- Size: 15 x 7"
- Finish: Clear anodized aluminum
- Distinctive: Cast aluminum construction with GM logo center cap
Heavy Duty (C20/K20)
Standard Steel Wheel:
- Size: 16 x 6.5"
- Bolt Pattern: 6 x 5.5"
- Load Rating: Increased to 3,200 lbs per wheel for 1981+
K5 Blazer Specific
1973-1980
- Standard: 15 x 7" steel wheel, 5 x 5" bolt pattern
- Rally option: Same as C10/K10 with RPO N67
1981-1987
- Standard: 15 x 7" steel wheel
- Aluminum option: Available 1982-1987 (RPO N90)
- Special editions: Some years featured unique wheel finishes
Suburban Variations
1/2-Ton Suburban (C10)
- Wheels: Same as C10 pickup specifications
- Load considerations: Higher load rating recommended due to vehicle weight
3/4-Ton Suburban (C20)
- Wheels: Same as C20 pickup specifications
- Mandatory: 6-lug pattern for all years
Tips & Best Practices
Restoration Considerations
Original Finish Restoration
- Steel wheels: Strip to bare metal, prime with epoxy primer, finish with correct color
- Rally wheels: Use argent silver automotive paint (PPG 4929 or equivalent)
- Trim rings: Polish stainless steel rings, replace damaged ones
Center Cap Authentication
Original GM center caps feature:
- Correct GM logo for year
- Proper mounting tabs
- Correct diameter for wheel application
Upgrading Considerations
Plus-Sizing Guidelines
- Maximum recommended: +1" diameter increase
- Width considerations: Ensure adequate clearance to suspension components
- Offset critical: Maintain similar offset to preserve handling characteristics
Load Rating Importance
- Never downgrade load rating from factory specification
- 3/4-ton applications: Minimum 3,000 lb rating required
- Towing applications: Consider higher rated wheels
Maintenance Best Practices
Regular Inspection Points
- Lug nut torque: Check every 5,000 miles
- 1/2-ton: 90-100 ft-lbs
- 3/4-ton: 120-140 ft-lbs - Wheel balance: Every 10,000 miles or when vibration occurs
- Visual inspection: Check for cracks, especially around lug holes
Troubleshooting
Vibration Issues
Symptoms: Steering wheel shake at highway speeds
Possible Causes:
- Wheel imbalance - Most common cause
- Bent rim - Check for visible damage
- Incorrect center bore - Lug-centric vs. hub-centric mounting
Solutions:
1. Professional balancing with road force measurement
2. Wheel straightening if bend is minor
3. Hub-centric rings if center bore is oversized
Clearance Problems
Symptoms: Rubbing noises, tire wear patterns
Check These Areas:
- Suspension components - Control arms, tie rods
- Brake calipers - Especially with aftermarket brakes
- Fender wells - At full steering lock
Solutions:
- Verify offset matches factory specification
- Check tire size compatibility
- Consider spacers only as last resort (affects handling)
Bolt Pattern Verification Issues
Symptoms: Wheels don't fit despite "correct" pattern
Common Problems:
- Metric vs. Imperial measurement confusion
- Aftermarket wheel tolerance issues
- Hub pilot diameter mismatch
Verification Steps:
1. Measure actual bolt circle with precision tools
2. Check center bore diameter
3. Test fit before final installation
Load Rating Concerns
Symptoms: Wheel cracking, failure under load
Prevention:
- Always verify wheel load rating meets or exceeds vehicle requirements
- Consider usage - towing, hauling, off-road
- Professional assessment for modified vehicles
Emergency Procedures:
- Immediate replacement of any cracked wheel
- Load reduction until proper wheels installed
- Professional inspection of remaining wheels
This information is compiled from GM service manuals, parts catalogs, and documented restoration practices. Always verify specifications with your specific vehicle's documentation and consult professionals for safety-critical applications.